💥 Nexus - Fix operation token on NexusWorkflowRunHandle

[Quinn-With-Two-Ns]

Fix operation token on NexusWorkflowRunHandle. Code had an obvious typo and more subtle url encoding issue. I also added some regression tests that test cross SDK token deserialization. Discussed among the Nexus team and we are okay making this change due to the low usage of dotnet Nexus and its current release stage.

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Note

Medium Risk
Changes the on-the-wire operation token format (base64url + corrected JSON key), which may break consumers relying on the previous encoding/typo despite added compatibility tests.

Overview
Fixes Nexus operation token serialization/deserialization in NexusWorkflowRunHandle by switching to base64url without padding and correcting the token JSON field name from t' to t.

FromToken now distinguishes invalid base64 vs invalid JSON (wrapping JsonException) and continues to reject unsupported non-zero versions; new unit tests cover round-trips, base64url formatting, version handling, invalid inputs, and cross-SDK (Python) token compatibility.

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